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The River Shannon ( or archaic an tSionna ) is the major river on the island of Ireland, and at in length, is the longest river in the British Isles. (2025). 9781840956993, EPA. ISBN 9781840956993 It drains the Shannon River Basin, which has an area of , – approximately one fifth of the area of Ireland.
Known as an important waterway since antiquity, the Shannon first appeared in maps by the Graeco-Egyptian geographer Ptolemy ( 100 – 170 AD). The river flows generally southwards from the Shannon Pot in County Cavan before turning west and emptying into the Atlantic Ocean through the long Shannon Estuary. Limerick city stands at the point where the river water meets the seawater of the estuary. The Shannon is tidal east of Limerick as far as the base of the Ardnacrusha dam.
From the Shannon Pot, the river subsumes a number of tributaries before replenishing Lough Allen at its head. The river runs through or between 11 of Ireland's counties, subsuming the tributary rivers Boyle, Inny, River Suck, Mulkear River and River Brosna, among others, before reaching the Shannon Estuary at Limerick.
Many different values have been given for the length of the Shannon. A traditional value is . An official Irish source gives a total length of (being fresh and tidal). Some Irish guides now give . Some academic sources give , although most will refuse to give a number. The reason is that there is no particular end to a river that empties into an estuary. The 344 km length relates to the distance between Shannon Pot and a line between Kerry Head and Loop Head, the furthest reaches of the land. (It also assumes the current shipping route via Ardnacrusha, which takes off the distance.) The 280 km distance finishes where the Shannon estuary joins the estuary of the River Fergus, close to Shannon Airport. Longer claimed lengths emerged before the use of modern surveying instruments.
At a total length of , it is the longest river in Ireland. That the Shannon is the longest river in the British Isles was evidently known in the 12th century, although a map of the time showed this river as flowing out of the south of Ireland.
There are some tributaries within the Shannon River Basin which have headwaters that are further in length (from source to mouth) than the Shannon Pot source's length of , such as the Owenmore River, total length in County Cavan and the Boyle River, total length with its source in County Mayo.
The River Shannon is a traditional freshwater river for about 45% of its total length. Excluding the tidal estuary from its total length of , if one also excludes the lakes (L. Derg , Lough Ree , Lough Allen plus Lough Boderg, L. Bofin, Lough Forbes, Lough Corry) from the Shannon's freshwater flow of , the Shannon, as a freshwater river, is only about long.
Apart from being Ireland's longest river, the Shannon is also, by far, Ireland's largest river by streamflow. It has a long-term average flow rate of (at Limerick). This is double the flow rate of Ireland's second highest-volume river, the short River Corrib (. If the discharges from all of the rivers and streams into the Shannon Estuary (including the rivers River Feale , River Maigue , River Fergus , and River Deel ) are added to the discharge at Limerick, the total discharge of the River Shannon at its mouth at Loop Head reaches . Indeed, the Shannon is a major river by the time it leaves Lough Ree with an average flow rate (at Athlone weir) of , larger than any of the other Irish rivers' total flow (apart from the River Corrib at Galway City).
Ptolemy's Geography (2nd century AD) described a river called Σηνος ( Sēnos) from PIE * sai-/ sei- 'to bind', the root of English sinew and Irish ‘’, referring to the long and sinuous estuary leading up to Limerick.
Vikings settled in the region in the 10th century and used the river to raid the rich monasteries deep inland. In 937 the Limerick Vikings clashed with those of Dublin on Lough Ree and were defeated.
In the 17th century, the Shannon was of major strategic importance in military campaigns in Ireland, as it formed a physical boundary between the east and west of the country. In the Irish Confederate Wars of 1641–53, the Irish retreated behind the Shannon in 1650 and held out for two further years against English Parliamentarian forces. In preparing a land settlement, or plantation after his conquest of Ireland Oliver Cromwell reputedly said the remaining Irish landowners would go to "Hell or Connacht", referring to their choice of forced migration west across the river Shannon, or death, thus freeing up the eastern landholdings for the incoming English settlers.
In the Williamite War in Ireland (1689–91), the Jacobitism also retreated behind the Shannon after their defeat at the Battle of the Boyne in 1690. Athlone and Limerick, cities commanding bridges over the river, saw bloody sieges (see Sieges of Limerick and Siege of Athlone).
As late as 1916, the leaders of the Easter Rising planned to have their forces in the west "hold the line of the Shannon". However, in the event, the rebels were neither well enough armed nor equipped to attempt such an ambitious policy.
The lower Shannon between Killaloe and Limerick had a topography quite different from the long upper reaches. Here the river falls by in only . William Ockenden, also from England, was placed in charge of works on this stretch in 1757 and spent £12,000 over the next four years, without fully completing the task. In 1771 parliament handed over responsibility to the Limerick Navigation Company, with a grant of £6,000 to add to their subscriptions of £10,000. A lateral canal, long with six locks, was started but the company needed more funds to complete it. In 1791, William Chapman was brought in to advise and discovered a sorry state of affairs – all the locks had been built to different dimensions and he spent the next three years supervising the rebuilding of most of them. The navigation was finally opened in 1799, when over of corn came down to Limerick, as well as and Peat. But even then, there were no in the river sections and there were still shoals in the summer months, as well as a lack of harbour facilities at Limerick, and boats were limited to load, often less.
With the approaching opening of the Grand Canal, the Grand Canal Company obtained permission from the Directors General of Inland Navigation, and asked John Brownrigg to do a survey which found that much of Omer's work had deteriorated badly, so they started repairs. After protracted negotiations on costs and conditions, the work was completed by 1810, so that boats drawing could pass from Athlone to Killaloe. Improvements on the lower levels were also undertaken, being completed by 1814.
When the Royal Canal was completed in 1817 there was pressure to improve the navigation above Lough Ree. The Jamestown Canal was repaired, harbours built and John Killaly designed a canal alongside the river from Battlebridge to Lough Allen, which was opened in 1820.
But the work the commissioners carried out failed to solve the problems of flooding and there were disastrous floods in the early 1860s. Given the flat nature of most of the riverbank, this was not easily addressed and nothing much was done until the 20th century.
In the 1950s traffic began to fall and low fixed bridges would have replaced opening bridges but for the actions of the Inland Waterways Association of Ireland which persuaded the Tánaiste to encourage passenger launches, which kept the bridges high enough for navigation. Since then the leisure trade has steadily increased, becoming a great success story.
Ballinasloe is linked to the Shannon via the River Suck and canal, while Boyle is connected via the Boyle canal, the river Boyle and Lough Key. There is also the Ardnacrusha canal connected with the Ardnacrusha dam south of Lough Derg. Near Limerick, a short canal connects Plassey with the Abbey River, allowing boats to bypass the Curraghower Falls, a major obstacle to navigation. Lecarrow village in County Roscommon is connected to Lough Ree via the Lecarrow canal. Jamestown Canal and the Albert Lock form a link between the River Shannon, from south of Jamestown, to Lough Nanoge to the south of Drumsna.
A small myth about Sionann tells that the legendary hunter-warrior Fionn mac Cumhaill was attacked by a number of other warriors at Ballyleague, near north Lough Ree. It is said that when Fionn was near to defeat, Sionnan rescued him, and he arrived with the Stone of Sionann, threw the stone, and the warriors were immediately killed. It further says that Fionn was afraid of the power of the stone and threw it into the river, where it remains at a low ford, and that if a woman named Be Thuinne finds it, then the world's end is near.
Shipping in the Shannon estuary was developed extensively during the 1980s, with over Irish pound2 billion (€2.5 billion) investment. A tanker terminal at Foynes and an oil jetty at Shannon Airport were built. In 1982 a large-scale alumina extraction plant was built at Aughinish. 60,000-tonne cargo vessels now carry raw bauxite from West African mines to the plant, where it is refined to alumina. This is then exported to Canada where it is further refined to aluminium. 1985 saw the opening of a 915 MW coal-fired electricity plant at Moneypoint, fed by regular visits by 150,000-tonne bulk carriers.
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